In recent years, fibrous sheetings have broadened the application to be made of them and have been widely employed in various industrial fields as industrial filters, battery separators, industrial wipers, etc.
Performance properties required for the sheets to be used as filters, separators or wipers include moderate permeability to air and liquids, proper strength, thickness and density, and high liquid retention. With the recent developments in industry, a demand for chemical resistance is also increasing. For example, sheets for use as filters for treating waste acids or waste alkalis or battery separators are required to exhibit chemical resistance enough to save themselves from elution or deterioration due to strong acids or strong alkalis. In particular, sheets for use as battery separators are demanded to have excellent alkali resistance in high temperatures as well as high liquid retention.
The sheet for use as battery separators have conventionally been prepared chiefly from polyamide fibers or polypropylene fibers. Sheets made of polyamide fibers, though excellent in liquid retention, have poor alkali resistance in high temperatures, which has been a cause of reducing a battery life through repetition of discharge and recharge. Sheets made of polypropylene fibers are excellent in alkali resistance in high temperatures but poor in liquid retention, thus causing reduction of a battery life similarly to the polyamide fibers.